Heat treatment of alloys having elements for improving grain boundary strength

ABSTRACT

Heat treatment of alloys having elements for improving grain boundary strength 
     Components directly after castings often reveal a low or no transverse grain boundary strength, so that cracks do appear and decrease the yield rate. 
     The inventive measures does not lead to low transverse grain boundary strength but maintains a efficient grain boundary strength, so that the yield rate of components without cracks is increased.

FIELD OF THE INVENTION

The present invention relates to a heat treatment of alloys, especiallynickel base superalloy and, more particularly, to castings having acolumnar grain microstructure.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,597,809 describes single crystal castings made from anickel base superalloy having a matrix with a composition consistingessentially of, in weight %, of 9.5% to 14% Cr, 7% to 11% Co, 1% to 2.5%Mo, 3% to 6% W, 1% to 4% Ta, 3% to 4% Al, 3% to 5% Ti, 6.5% to 8% Al+Ti,0% to 1% Nb, and balance essentially nickel with the matrix containingabout 0.4 to about 1.5 volume of a phase based an tantalum carbide as aresult of the inclusion in the alloy of about 0.05% to about 0.15% C andextra Ta in an amount equal to 1 to 17 times the C content.

Single crystal castings produced from the aforementioned nickel basesuperalloy exhibit inadequate transverse grain boundary strength. Thepresent inventors attempted to produce directionally solidified (DS)columnar grain castings of the nickel base superalloy. However, thedirectionally solidified (DS) columnar grain castings produced wereunacceptable as DS castings as a result of the castings exhibitingessentially no transverse grain boundary strength and no ductility whentested at a temperature of 750 degrees C. (1382 degrees F.) and stressof 660 MPa (95.7 Ksi). The transverse grain boundary strength andductility were so deficient as to render DS columnar grain castingsproduced from the aforementioned nickel base superalloy unsuitable foruse as turbine blades of gas turbine engines.

WO 99/67435 discloses nickel base superalloy castings having boron addedto improve transverse stress rupture strength and ductility of DScastings. The castings are heat treated at 1250° C. for 4 h so that afully solution of the secondary phase (γ′-phase) is performed. Due tothe occurrence of grain boundary cracks after the fully solution heattreatment the producibility is so deficient as to render DS columnargrain castings produced from the aforementioned nickel base superalloyunsuitable for use as turbine blades of gas turbine engines.

An object of the present invention is to provide a heat treatment ofalloys, especially of as-cast alloys, e.g. DS columnar grain castingsbased on the aforementioned single crystal nickel base superalloy,having substantially improved transverse stress rupture strength andductility as well as producibility to an extent that the DS castings areacceptable for use as high temperature applications such as turbineblades of a gas turbine engine.

SUMMARY OF THE INVENTION

The present invention involves a heat treatment of cast alloys, such assuperalloys, having at least one addition, which improves grain boundarystrength such as boron in the nickel base superalloy described hereabove in a manner discovered to significantly improve transverse stressrupture strength and ductility of directionally solidified (DS) columnargrain castings produced with a heat treatment, which solves a secondaryphase only partly, e.g. no fully solution heat treatment is performed.

Boron is often added to superalloy compositions in an effective amountto substantially improve transverse stress rupture strength andductility of directionally solidified columnar grain castings producedfrom the boron-modified superalloy. The boron concentration preferablyis controlled in the range of about 0.003% to about 0.0175% by weight ofthe superalloy composition to this end.

In conjunction with addition of boron to the superalloy composition, thecarbon concentration preferably is controlled in the range of about0.05% to about 0.11% by weight of the superalloy composition.

A preferred nickel base superalloy in accordance with an embodiment ofthe present invention consists essentially of, in weight %, of about11.6% to 12.70% Cr, about 8.50% to 9.5% Co, about 1.65% to 2.15% Mo,about 3.5% to 4.10% W, about 4.80% to 5.20% Ta, about 3.40% to 3.80% Al,about 3.9% to 4.25% Ti, about 0.05% to 0.11% C, about 0.003% to 0.0175%B, and balance essentially Ni. The boron modified nickel base superalloycan be cast as DS columnar grain castings pursuant to conventional DScasting techniques such as the well known Bridgman mould withdrawaltechnique.

DS castings produced in this manner typically have a plurality ofcolumnar grains extending in the direction of the principal stress axisof the casting with the <001> crystal axis generally parallel to theprincipal stress axis. DS columnar grain castings pursuant to thepresent invention preferably exhibit a stress rupture life of at leastabout 100 hours and elongation of at least about 2.5% when tested at atemperature of 750 degrees C. (1382 degrees F.) and stress of 660 MPa(95.7 Ksi) and will find use as turbine blades, vanes, outer air sealsand other components of a industrial and aero gas turbine engines.

The above objects and advantages of the present invention will becomemore readily apparent form the following detailed description taken withthe following drawings.

DETAILED DESCRIPTION OF THE INVENTION

Exemplarily as alloy a nickel base superalloy is chosen which consistsessentially of, in weight %, of about 9.5% to 14% Cr, about 7% to 11%Co, about 1% to 2.5% Mo, about 3% to 6% W, about 1% to 6% Ta, about 3$to 4% Al, about 3% to 5% Ti, about 0% to 1% Nb, and balance essentiallyNi and B present in an amount effective to substantially improvetransverse stress rupture strength of a DS casting as compared to asimilar casting without boron present.

The inclusion of boron, as an addition, which improves the grainboundary strength in the alloy, is chosen in an amount discoveredeffective to provide substantial transverse stress rupture strength andductility of a DS columnar grain casting produced from the alloy ascompared to a similar casting without boron present.

Preferably, the nickel base superalloy is modified by the inclusion ofboron B in the range of about 0.003% to about 0.0175%, preferably 0.010%to 0.015%, by weight of the superalloy composition to this end.

In conjunction with addition of boron to the superalloy composition, thecarbon C concentration is controlled in a preferred range of about 0.05%to about 0.11% by weight of the superalloy composition. Also SiliziumSi, Zirkonium Zr and Hafnium Hf can be used as addition.

Furthermore all combinations of B, C, Si, Zr, Hf are possible.

The transverse stress rupture strength and ductility as well as theproducibility of DS castings produced from nickel base superalloy withthe modified heat treatment are provided to an extent that the castingsare rendered acceptable for use as turbine blades and other componentsof gas turbine engines.

A particularly preferred boron-modified nickel base superalloy castingcomposition consists essentially of, in weight %, of about 11.6% to12.70% Cr, about 8.5% to 9.5% Co, about 1.65% to 2.15% Mo, about 3.5% to4.10% W, about 4.80% to 5.20% Ta, about 3.40 to 3.80% Al, about 3.9% to4.25% Ti, about 0.05% to 0.11% C, about 0.003% to 0.0175% B, and balanceessentially Ni and castable to provide a DS columnar grainmicrostructure.

The DS microstructure of the columnar grain casting typically includesabout 0.4 to about 1.5 volume % of a phase based an tantalum carbide.

Although not wishing to be bound by any theory, it is thought that boronand carbon tend to migrate to the grain boundaries in the DSmicrostructure to add strength and ductility to the grain boundaries athigh service temperatures, for example 816 degrees C. (1500 degrees F.)typical of gas turbine engine blades. DS columnar grain castingsproduced from the above boron modified nickel base superalloy typicallyhave the <001> crystal axis parallel to the principal stress axis of thecasting and exhibit a stress rupture life of at least about 100 hoursand elongation of at least about 2.5% when tested at a temperature of750 degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi)applied perpendicular to the <001> crystal axis of the casting.

For example, the following DS casting tests were conducted and areoffered to further illustrate, but not limit, the present invention.

A heat #1 having a nickel base superalloy composition in accordance withthe aforementioned U.S. Pat. No. 4,597,809 and heats #1A and #2 and #3of boron modified nickel base superalloy were prepared with thefollowing compositions, in weight percentages, set forth in Table I:

TABLE I Heat Cr Co Mo W Ta Al Ti C B Ni #1 12.1 9.0 1.8 3.7 5.2 3.6 4.00.07 0.001 balance #1A 12.1 9.0 1.8 3.7 5.2 3.6 4.0 0.08 0.010 balance#2 12.1 9.0 1.8 3.7 5.2 3.6 4.0 0.09 0.011 balance #3 12.1 9.0 1.8 3.75.2 3.6 4.0 0.08 0.014 balance

Each heat was cast to form DS columnar grain non-cored castings having arectangular shape for transverse stress rupture testing pursuant toASTM-E-139 testing procedure. The DS castings were produced e.g. usingthe conventional Bridgman mould withdrawal directional solidificationtechnique.

For example, each heat was melted in a crucible of a conventionalcasting furnace under a vacuum of 1 micron and superheated to 1427degrees C. (2600 degrees F.). The superheated melt was poured into aninvestment casting mould having a face coat comprising zircon backed byadditional slurry/stucco layers comprising zircon/alumina. The mould waspreheated to 1482 degrees C. (2700 degrees F.) and mounted an a chillplate to effect unidirectional heat removal from the molten alloy in themould. The melt-filled mould an the chill plate was withdrawn from thefurnace into a solidification chamber of the casting furnace at a vacuumof 1 micron at a withdrawal rate of 6-16 inches per hour.

The DS columnar grain castings were cooled to room temperature undervacuum in the chamber, removed from the mould in conventional mannerusing a mechanical knock-out procedure, heat treated at a temperatureand for a duration in such way, that the solution of a secondary phasein the matrix is only partly performed.

The nickel based superalloy has as a secondary phase the γ′-phase.

For a specimen (e.g. nickel based superalloy) with the composition ofclaim 21, the inventive heat treatment is performed after casting at1213° C. for at least 1 h, which is not the solution temperature of asecondary phase (e.g. y′ phase) for this alloy.

Also the temperature of 1250° C. (called fully solution temperature),which is normally used for a fully solution treatment, can be used butonly as long as the secondary phase is not completely solved in thematrix.

The not solubilized amount of the secondary phase in the matrix issmaller than 90, 70, 50 or 30 vol % according to the geometry andproducibility after the heat treatment, because grain boundary cracksare avoided, in order to increase the yield rate of specimens anddesired mechanical properties of the specimen.

The alloy can have a single crystal structure or only having grainsalong one direction.

Optionally an ageing heat treatment can be performed for thiscomposition at 1080° C. for at least 2 h after this solution heattreatment. Optionally followed by a second ageing heat treatment at 870°C. for at least 12 h.

Especially the inventive heat treatment is used for hollow specimen,especially blades, vanes, or liners because cracks do appear more oftenin walls, especially in thin walls, than in massive specimens after thenormally used heat treatment after casting.

The inventive heat treatment leads to an increased grain boundarystrength during this heat treatment, so that the yield rate (componentswithout cracks) after the heat treatment is increased.

Also the transverse stress rupture of the component as final product isincreased during use of the component at working conditions, becausegrain boundary strength is increased. The inventive method yields alsogood results for massive components, e.g. of a gas turbine.

The castings were also analysed for chemistry, and machined to specimenconfiguration.

Stress rupture testing was conducted in air at a temperature of 750degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi) appliedperpendicular to the <001>crystal axis of the specimens.

The results of stress rupture testing are set forth in TABLE II belowwhere LIFE in hours (HRS) indicates the time to fracture of thespecimen, ELONGATION is the specimen elongation to fracture, and RED OFAREA is the reduction of area of the specimens to fracture. The BASELINEdata corresponds to test data for Heat #1, and the #1A, #2 and #3 datacorresponds to test data for Heat #1A, #2 and #3, respectively. TheBASELINE data represent an average of two stress rupture test specimens,while the #1A, #2 and #3 data represent a single stress rupture testspecimen.

TABLE II Temperature Stress Elon- Red of #of ° C. MPa Life gation AreaAlloy Tests (° F.) (KSI) (HRS) (%) (%) Baseline 2 750 (1382) 660 (95.7)0 0 0 #2 1 750 (1382) 660 (95.7) 182 2.6 6.3 #3 1 750 (1382) 660 (95.7)173 3.7 10.7 #1A 1 750 (1382) 660 (95.7) 275 3.1 4.7

It is apparent from TABLE II that the DS columnar grain specimensproduced from heat #1 exhibited in effect essentially no (e.g. zerohours stress rupture life) transverse grain boundary strength whentested at a temperature of 750 degrees C. (1382 degrees F.) and stressof 660 MPa (95.7 Ksi). That is, the specimens failed immediately toprovide an essentially zero stress rupture life. Moreover, theelongation and reduction of area data were essentially zero. Thesestress rupture properties are so deficient as to render the DS columnargrain castings produced from heat #1 unacceptable for use as turbineblades of gas turbine engines.

In contrast, TABLE II reveals that DS columnar grain specimens producedfrom heat #1A exhibited a stress rupture life of 275 hours, anelongation of 3.1%, and a reduction of area of 4.7 and specimens fromheat #2 exhibited a stress rupture life of 182 hours, an elongation of2.6%, and a reduction of area of 6.3% when tested at a temperature of750 degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi). Thesestress rupture properties of the invention represent an unexpected andsurprising improvement over those of specimens produced from heat #1 andrender DS columnar grain castings produced from heats #1A, #2 and #3more suitable for use as turbine blades and other components of gasturbine engines.

The present invention is effective to provide DS columnar grain castingswith substantial transverse stress rupture strength and ductility. Theseproperties are achieved without adversely affecting other mechanicalproperties, such as tensile strength, creep strength, fatigue strength,and corrosion resistance of the DS castings. The present invention isespecially useful to provide large DS columnar grain industrial gasturbine (IGT) blade castings which have the alloy composition describedabove to impart substantial transverse stress rupture strength andductility to the castings and which have a length of about 20centimeters to about 60 centimeters and above, such as about 90centimeters length, used throughout the stages of the turbine ofstationary industrial gas turbine engines. The above describedboron-modified nickel base superalloy casting composition can be cast asDS columnar grain or single crystal components.

While the invention has been described in terms of specific embodimentsthereof, it is not intended to be limited thereto but rather only to theextent set forth in the following claims.

1. A method of heat treating a directionally solidified or singlecrystal nickel based cast alloy comprising boron which improves grainboundary strength, wherein the alloy has a secondary phase aftercasting, which can be solved in the matrix of the alloy at a solutiontemperature, wherein a heat treatment is performed in such a way thatthe secondary phase is only partly solved.
 2. A method of claim 1,wherein at least one aging treatment is performed after the heattreatment.
 3. A method of claim 1, wherein the temperature of the heattreatment is less than the fully solution temperature.
 4. A method ofclaim 1, wherein the duration of the heat treatment is chosen in such away, that the secondary is not completely solved.
 5. (canceled)
 6. Amethod of claim 1, wherein the heat treatment is performed with hollowcomponents.
 7. A method of claim 6, wherein the heat treatment isperformed with components having a length of about 20 centimeters toabout 60 centimeters.
 8. A method of claim 6, wherein the heat treatmentis performed with hollow components having a thickness of an outer wallsmaller than 8 mm.
 9. A method of claim 5, wherein the secondary phaseis the γ′-phase.
 10. (canceled)
 11. A method of claim 1, wherein theheat treatment is performed with an alloy having carbon as an addition.12. A method of claim 1, wherein the heat treatment is performed with analloy having a directionally solidified columnar grains.
 13. A method ofclaim 1, wherein the heat treatment is performed with an alloy having asingle crystal structure.
 14. A method of claim 1, wherein theparameters of the heat treatment is chosen in such a way that the amountof the secondary phase brought into solution is smaller than 90 vol %.15. A method of claim 1, wherein the parameters of the heat treatment ischosen in such a way that the amount of the secondary phase brought intosolution is smaller than 70 vol %.
 16. A method of claim 1, wherein theparameters of the heat treatment is chosen in such a way that the amountof the secondary phase brought into solution is smaller than 50 vol %.17. A method of claim 1, wherein the parameters of the heat treatment ischosen in such a way that the amount of the secondary phase brought intosolution is smaller than 30 vol %.
 18. A method of claim 1, wherein theheat treatment is performed with a directionally solidified columnargrain nickel base alloy casting, consisting essentially of, in weight %of about 9.5% to 14% Cr, about 7% to 11% Co, about 1% to 2.5% Mo, about3% to 6% W, about 1% to 6% Ta, about 3% to 4% Al, about 3% to 5% Ti,about 0% to 1% Nb, and balance essentially Ni and B present in an amounteffective to substantially improve transverse stress rupture strength ofsaid casting as compared to a similar casting without boron present. 19.A method of claim 18, wherein the heat treatment is performed with analloy, wherein B is present in the range of about 0.003% to about 0.018%by weight.
 20. A method of claim 18, wherein the alloy after the heattreatment has a stress rupture life of at least about 100 hours andelongation to fracture of at least about 2.5% when tested at atemperature of 750 degrees C. (1382 degrees F.) and stress of 660 MPa(95.7 Ksi) applied in a direction perpendicular to a <001> crystal axisof said casting.
 21. A method of claim 1, wherein the heat treatment isperformed with a directionally solidified columnar grain nickel basealloy casting consisting essentially of, in weight %, of about 11.6% to12.70% Cr, about 8.5% to 9.5% Co, about 1.65% to 2.15% Mo, about 3.5% to4.10% W, about 4.8% to 5.20% Ta, about3.4% to 3.80% Al, about 3.9% to4.25% Ti, about 0.05% to 0.11% C, about 0.003% to 0.0175% B, balanceessentially Ni and having substantially improved transverse stressrupture strength as compared to a similar casting without boron present.22. A method of claim 21, wherein the alloy after the heat treatment hasa stress rupture life of at least about 120 hours and elongation of atleast about 2.5% when tested at a temperature of 750 degrees C. (1382degrees F.) and stress of 660 MPa (95.7 Ksi) applied perpendicular to a<001> crystal axis of said casting.
 23. A method of claim 1, wherein theheat treatment is performed with directionally solidified columnar grainnickel base alloy casting having a nominal composition consistingessentially of, in weight %, of about 12.00% Cr, about 9.00% Co, about1.85% Mo, about 3.700% W, about 5.10% Ta, about 3.60% Al, about 4.00%Ti, about 0.0125% B, about 0.09% C, balance essentially Ni and having astress rupture life of at least about 100 hours and elongation tofracture of at least about 2.5% when tested at a temperature of 750degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi) appliedperpendicular to a <001> crystal axis of said casting.
 24. A method ofclaim 1, wherein the heat treatment is performed after casting.
 25. Amethod of claim 4, wherein the fully solution temperature is used.
 26. Amethod of claim 6, wherein the hollow components are chosen of the groupconsisting of vanes, blades and liners.
 27. A method of claim 1, whereinthe heat treatment is performed with massive components.
 28. A method ofclaim 1, wherein the heat treatment is performed with an alloy having anaddition selected from the group consisting of Zircon, Silicon, Hafnium.29. A method of claim 3, wherein the duration of the heat treatment ischosen in such a way, that the secondary is not completely solved.